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Five Unit Code
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By Alan G Hobbs, G8GOJ
President of British Amateur Radio Teledata Group, (c) 1999
In articles that mention RTTY codes there is usually reference to Baudot,
Murray and ITA2 codes. These codes are often taken to be identical and interchangeable.
Even "respectable" engineering journals do not seem to understand
the fundamental differences between the different codes. For any two equipments
to satisfactorily inter-operate, it is essential that the code in use is
thoroughly specified and understood, and the same at each end. The purpose
of this article is to explain the similarities, and the differences, between
the codes, and to indicate their relationship to the Radio Amateur.
All codes have their strengths and their weaknesses. For instance, one
of the strengths of Morse code is that commonly used letters have short
codes, making them easier to send. Whereas one of its weaknesses is the
difference in length between the code for the shortest character E,
and the code for the longest character 0, which takes 19 times
as long to transmit. This vast difference in length made the Morse code
difficult, but certainly not impossible, to mechanise. For example, the
Creed Morse printer, developed in the early 1900s, read and printed in
plain language, a perforated Morse tape at speeds of up to 100 words per
It had long been realised by many telegraphic engineers, that the real
answer to the mechanisation of telegraphy was to use a code in which every
character took the same time to transmit. A so-called "constant length"
code. With 26 letters in the alphabet, it was only natural that the most
popular codes would all consist of five signalling elements, with each
element taking one of two states, e.g. +ve/-ve, off/on, etc. Therefore
the number of available combinations is two raised to the power five:
ie 2 x 2 x 2 x 2 x 2 = 32
By reserving two of the combinations for use as non-printing shift control
characters, it is possible to associate a numeral or punctuation mark
with every letter of the alphabet, effectively doubling the capacity of
the code. Naturally, this will slightly reduce the rate at which the message
is transmitted, but the machinery could be designed to insert these shift
characters automatically, thereby reducing the effort on the part of the
Baudot Multiplex System
The earliest, successful, printing telegraph system which used a five-unit
code, was the Baudot Multiplex System, which was devised by Emile Baudot,
of the French Telegraphic Service, in 1874. This is a time division multiplex
system, and utilises (1) certain printing details of the Hughes printing
telegraph instrument, (2) the distributor arrangements invented by Bernard
Meyer in 1871 which were employed in a Morse multiplex system, and (3)
a five-unit code devised by Johann Gauss and Wilhelm Weber. The system
was adopted in France in 1877, and thereafter its use in France was extensive,
and it was to some extent adopted in other countries. The British Post
Office adopted the Baudot system for use on a simplex circuit between
London and Paris in 1897, and subsequently made considerable use of duplex
Baudot systems on their Inland Telegraph Services.
The Baudot distributor could be designed so that it could be used by
from two to six operators, with the quadruple Baudot system, using four
operators, adopted as the standard installation for use in the British
Post Office. The distributor, consisting of copper segments and rotating
brushes, successively connected each operator to the line, for a time
long enough to transmit the five units corresponding to one character.
Additional segments transmitted correcting currents, from one end to the
other, to maintain synchronism between the sending and receiving stations.
Hence the Baudot system was one of the earliest five-unit synchronous
The standard speed of transmission, by each operator, was 180 characters
per minute, each character being set-up manually on a small piano-like
keyboard, which only had five keys. The keys were so arranged that once
pressed down, they latched down, and were only released by the distributor
when all the five elements of the character had been transmitted.
The operator was given an audible indication of the keyboard unlocking
by means of what is known as the "cadence signal". This signal
came from the operation of the electromagnet which released the keys.
The manipulation of the Baudot keyboard called for a high degree of operating
skill, since a definite, unvarying, rhythmic speed of signalling was necessary.
Figure 1 shows the allocation of the Baudot code which was employed
in the British Post Office for continental and inland services. It will
be observed that a number of characters in the continental code are replaced
by fractionals in the inland code. Code elements 1, 2 and 3 are transmitted
by keys 1, 2 and 3, and these are operated by the first three fingers
of the right hand. Code elements 4 and 5 are transmitted by keys 4 and
5, and these are operated by the first two fingers of the left hand.
Because the combinations were set-up manually, the code was so arranged
that the finger movements to be performed by the operator were as evenly
divided as possible between the right and left hands, and also as few
as possible for those characters having the greatest frequency of occurrence.
This ensured the minimum fatigue of the operator.
A fine example of Baudot equipment may be seen in the Science Museum
in London. Until the autumn of 1997, another fine example was to be seen
in the BT Museum in London. Unfortunately, this museum is now closed to
The Baudot code was eventually standardised for multiplex systems as
the International Telegraph Alphabet number 1 (ITA1), and is shown in
Figure 2. International Telegraph Alphabet Number 1
Murray Type Printing Multiplex System
This system was designed in 1901 by Donald Murray, a New Zealand sheep
farmer, as a combination of the best features of the Baudot multiplex
system and the Murray automatic system. Murray also employed a five-unit
code, but the allocations of the of the signal combinations differed very
considerably from that used in the Baudot code, as is shown in figure
Figure 3. The Murray Code
The main reason for this was that he choose to use a keyboard layout
similar to that of a typewriter, which relieved the operator of the burden
of setting up the individual code elements. This allowed Murray to allocate
the codes so that those characters having the greatest frequency of occurrence
were given a combination which involved the least number of mechanical
operations, thereby reducing the wear in the equipment.
At the transmitting end, the Murray system comprised: (1) A keyboard
perforator, which produced a tape in which the code was perforated transversely.
The feed holes being in line with the front edges of the perforations,
so that the direction in which the tape should be read was at once apparent,
and; (2) A transmitter which could be mounted adjacent to the perforator
in order to give the minimum possible distance between the perforating
and transmitting mechanisms. With this arrangement the distance was reduced
to only 16 character spaces.
In the transmitter, the five contact levers which sensed the perforations
in the tape were connected to individual segments on a distributor, very
similar in principle to the Baudot transmitter distributor. Additional
segments on the distributor operated an electromagnet which stepped the
tape forward after the line brush had passed the segments connected to
the five contact levers. A novel feature on the transmitter was a start-stop
device which sensed the size of the tape loop between the perforator and
the transmitter, and held the five sensing levers in the space position,
thereby sending spacing currents to line until the tape became slack.
Mutilation of the tape, or disconnection of the transmitter, was thus
At the receiving end, the Murray system comprised: (1) A reperforator
which produced perforated tape corresponding to the original sending tape,
and which could then be used for onward transmission to further stations,
and; (2) A printing receiver which interpreted the incoming line signals,
and printed the characters on a paper tape. The Creed multiplex printer
was commonly used for this purpose, which employed a series of bell-cranks
and a rotating typehead, as used on the later models 3 and 7 series of
teleprinters. Either the reperforator, the printing receiver, or both,
could be connected to the receiving distributor as required by the local
Synchronous printing telegraph systems employing constant length codes,
such as the Baudot and Murray, were a great advance over the previous
telegraph systems. However, they suffered from a lack of flexibility,
and required very accurate means for maintaining accurate synchronism
between the transmitting and receiving instruments. To overcome these
disadvantages, a number of inventors experimented with the idea of starting
and stopping the receiving mechanism for each character. For this purpose,
a "start" signal was transmitted immediately preceding the code
elements, and a "stop" signal was transmitted immediately the
code elements had been transmitted.
The code employed was still a five- unit code, with the start signal
equal in duration to one code element, and the stop signal being in some
cases equal in duration to one code element, and in other case more than
one element often 1.5 elements. For this reason the code is sometimes
referred to as a 7½ unit code. The transmitting and receiving instruments
were now arranged to have a definite rest position, at which point they
were precisely in phase with each other in readiness for their respective
timing cycles when released.
Because the transmitter and receiver effectively re-synchronised at
the start of each character, it was no longer necessary for the speed
of the instruments to be very accurately controlled, and simpler centrifugal
governors which maintained the speed to within +/- 0.5% were now adequate.
This implies the possibility of a noticeable speed difference between
the two ends of a system, so the receiving mechanism is arranged to rotate
for a shorter time period than the transmitter mechanism. The time difference
usually being equal to one element period, but sometimes only equal to
half of one element period. By this means the receiver was always at rest
before the start of the next character, even with speed errors greater
The earliest type of start-stop instrument was introduced in America
in 1907 by Charles L Krumm and his son H Krumm. It was manufactured by
the Morkrum company, which would later become the Teletype corporation,
and began to find practical application about 1920. The instrument employed
a typewriter style keyboard, and printed the received signals direct onto
paper tape, without requiring the intermediate use of perforated tape
at either end of the system. It was capable of working at a speed of 40
words per minute, in either simplex or duplex.
In 1922, Frederick George Creed in Croydon designed a start-stop receiver,
and a few years later produced a combined transmitter and receiver having
a typewriter-style keyboard. This machine, known as the Model 3 and operating
at 65.3 words per minute, printed the messages directly onto a gummed
paper tape and was widely adopted for the British Post Office Public Telegram
service. The year 1931 saw the introduction of the first Creed Model 7
page printing teleprinter, operating at the now standard speed of 66.6
words per minute.
Early start-stop machines tended to use versions of the Murray code
but, in the 1930s, the CCITT standardised on the International Telegraph
Alphabet number 2 (ITA2), shown in figure 4, for start-stop telegraph
systems. The Americans chose to use a variation of ITA2 known as the Teletypewriter
code, which is shown in figure 5.
Figure 4. International Telegraph Alphabet number 2
Figure 5. Teletypewrite Code
Virtually all mechanical teleprinter equipment which remains in Amateur
hands dates from after the early 1930s and was, therefore, designed in
accordance with CCITT standards, and uses either ITA2 or its American
The only teleprinters which used the Murray code, and may still
exist in ever deceasing numbers, are the very early Creed models 3A, 3W,
3X, 3Y and 3Z tape printing machines. The later Creed models 3B, 3C, 3D
and 3E used the standard ITA2 code.
No teleprinters were ever produced which used the Baudot code, but that
is hardly surprising when one considers that the Baudot code was used
in a very early synchronous system, and all teleprinters, as we now know
them, operate on the start-stop (asynchronous) principle. Also, as far
as this writer is aware no computer programmer has yet implemented the
Baudot code or the Murray code for the Amateur home computer market, no
matter what may be found in advertisements in the Amateur press. For those
readers who wish to learn more about the history of telegraphic communications,
and the ingenuity of the engineers and inventors, this writer would recommend
a trip to a library, where you should ask for: Telegraphy by J W Freebody,
published by Sir Isaac Pitman in 1958.
Reprinted by permission of Alan Hobbs via Sam Hallas - 3/5/99.